VGCF has excellent material properties of a unique onion-ring micro-structure, a high aspect ratio, a high thermal-conductive coefficient, a low thermal-expansion coefficient, high strength, high modulus and high corrosion resistance. In addition, carbon fibers made by the vapor-grown method can have the structure similar to the single-crystal graphite structure, thereby forming excellent multi-wall carbon tubes having excellent electrical conductivity, wherein the thermal conductivity thereof is better than that of the thermally-conductive material such as copper or aluminum. The success of VGCF study has added quite an important technique to the carbon fiber industry in which OPCF (Organic Precursor Carbon Fibers) such as PAN, Pitch carbon fibers have been used mainly quite a long time.
The VGCF technique mainly uses low hydrocarbon compounds as raw material (carbon source) having pyrolysis reaction in reductive carrier gas (H2) atmosphere, thus directly forming VGCF via the special catalysis of transition metals such as iron, nickel or cobalt by using ultra-fine nano-particles thereof as nucleation, wherein the reaction temperature is between 800° C. and 1300° C. The VGCF fabrication technique has the advantage that the fabrication skill is simple and does not need to perform the steps of spinning, pre-oxidation, carbonization, etc. required in the OPCF fabrication technique, so that the VGCF fabrication technique can form carbon fibers directly from cheap low-hydrocarbon material via pyrolysis and catalysis.
Conventional vapor-grown carbon fibers (VGCF) are generally formed in linear one-dimensional structures, or very few of them are the products having two-dimensional structures such as T-shaped or Y-shaped structures. In comparison with one-dimensional or two-dimensional structures, the VGCF with three-dimensional structures can further enhance the properties of strength, electrical and thermal conductivities for the composite materials made thereof. Therefore, the conventional methods for fabricating the VGCF with three-dimensional structures are, for example, Taiwan Patent Publication No. 552156 is dedicated to growing the VGCF having more T-shaped or Y-shaped branched structures on a substrate; and Japan Patent Publication No. 2004-119386 suggests converting the VGCF with one-dimensional structures to the VGCF with three-dimensional (linkage) structures by attaching asphalt to one-dimensional VGCF and then performing special treatments such as carbonizing. However, the former is restricted to the use of substrate and is difficult for mass production, and the latter has to perform additional treatments after forming the VGCF, thus having the disadvantages of increased production cost and lowered production efficiency and product performance.
Hence, there is a need to develop an effective method for directly and continuously producing VGCF having a three-dimensional linkage structure with no additional treatments needed, thus promoting production efficiency; lowering production cost; and benefiting mass production.